fix(grpc): check proxy list length in policies (#7512)

plugin/grpc/policy.go

@@ -20,6 +20,8 @@ func (r *random) String() string { return "random" }
 
 func (r *random) List(p []*Proxy) []*Proxy {
 	switch len(p) {
+	case 0:
+		return nil
 	case 1:
 		return p
 	case 2:
@@ -46,6 +48,9 @@ type roundRobin struct {
 func (r *roundRobin) String() string { return "round_robin" }
 
 func (r *roundRobin) List(p []*Proxy) []*Proxy {
+	if len(p) == 0 {
+		return nil
+	}
 	poolLen := uint32(len(p))
 	i := atomic.AddUint32(&r.robin, 1) % poolLen
 

plugin/grpc/policy_test.go

@@ -0,0 +1,128 @@
+package grpc
+
+import (
+	"testing"
+)
+
+func TestRoundRobinEmpty(t *testing.T) {
+	t.Parallel()
+
+	r := &roundRobin{}
+	got := r.List(nil)
+	if len(got) != 0 {
+		t.Fatalf("expected length 0, got %d", len(got))
+	}
+}
+
+func TestRandomEmpty(t *testing.T) {
+	t.Parallel()
+
+	r := &random{}
+	got := r.List(nil)
+	if len(got) != 0 {
+		t.Fatalf("expected length 0, got %d", len(got))
+	}
+}
+
+func TestSequentialEmpty(t *testing.T) {
+	t.Parallel()
+
+	r := &sequential{}
+	got := r.List(nil)
+	if len(got) != 0 {
+		t.Fatalf("expected length 0, got %d", len(got))
+	}
+}
+
+func TestPoliciesOrdering(t *testing.T) {
+	t.Parallel()
+
+	p0 := &Proxy{addr: "p0"}
+	p1 := &Proxy{addr: "p1"}
+	p2 := &Proxy{addr: "p2"}
+	in := []*Proxy{p0, p1, p2}
+
+	t.Run("sequential keeps order", func(t *testing.T) {
+		t.Parallel()
+
+		r := &sequential{}
+		got := r.List(in)
+		if len(got) != len(in) {
+			t.Fatalf("expected length %d, got %d", len(in), len(got))
+		}
+		for i := range in {
+			if got[i] != in[i] {
+				t.Fatalf("sequential order changed at %d: want %p, got %p", i, in[i], got[i])
+			}
+		}
+	})
+
+	t.Run("round robin advances and permutation", func(t *testing.T) {
+		t.Parallel()
+
+		r := &roundRobin{}
+
+		got1 := r.List(in)
+		if !isPermutation(in, got1) {
+			t.Fatalf("first call: expected permutation of input")
+		}
+		if got1[0] != p1 {
+			t.Fatalf("first element should advance to p1, got %p", got1[0])
+		}
+
+		got2 := r.List(in)
+		if !isPermutation(in, got2) {
+			t.Fatalf("second call: expected permutation of input")
+		}
+		if got2[0] != p2 {
+			t.Fatalf("first element should advance to p2 on second call, got %p", got2[0])
+		}
+
+		got3 := r.List(in)
+		if !isPermutation(in, got3) {
+			t.Fatalf("third call: expected permutation of input")
+		}
+		if got3[0] != p0 {
+			t.Fatalf("first element should wrap to p0 on third call, got %p", got3[0])
+		}
+	})
+
+	t.Run("random is a permutation", func(t *testing.T) {
+		t.Parallel()
+
+		r := &random{}
+		got := r.List(in)
+		if !isPermutation(in, got) {
+			t.Fatalf("random did not return a permutation of input")
+		}
+	})
+
+	t.Run("random with two proxies", func(t *testing.T) {
+		t.Parallel()
+
+		r := &random{}
+		in2 := []*Proxy{p0, p1}
+		got := r.List(in2)
+		if !isPermutation(in2, got) {
+			t.Fatalf("random did not return a permutation of input")
+		}
+	})
+}
+
+// Helper: returns true if b is a permutation of a (same multiset of pointers).
+func isPermutation(a, b []*Proxy) bool {
+	if len(a) != len(b) {
+		return false
+	}
+	count := make(map[*Proxy]int, len(a))
+	for _, p := range a {
+		count[p]++
+	}
+	for _, p := range b {
+		count[p]--
+		if count[p] < 0 {
+			return false
+		}
+	}
+	return true
+}